A number of attempts have been made in order to improve the yield of plants. Among them, studies on plant growth regulators have been rapidly developed in recent years since plant hormones, which are physiologically active substances in common to all plants, were discovered. Six plant hormones, namely, gibberellin, auxin, cytokinin, ethylene, abscisic acid and brassinolide are known at present.
However these plant hormones mainly affect only a part of a plant organ. For example, indoleactic acid promotes rooting, gibberellin is usable in the formation of seedless grapes, ethephon promotes maturing of fruits and maleic hydrazide is usable as a sucker inhibitor for tobacco. Thus, none of them affects the whole plant or increases its yield.
On the other hand, chemicals improving the photosynthetic capability of a plant have attracted public attention since they affect the whole plant and thus increase its yield. For example, it has been found that N-allyl-N-methylglycine and N,N-dimethylglycine improve the photosynthetic capability in cultured cells (refer to Proceedings of Society of Plant Chemical Regulation in 1990). It is known, further, that choline chloride and its derivatives improve photosynthetic capability, though this function is still unsatisfactory.